There is a desire to decrease carbon dioxide (CO2) emissions from industrial facilities and power plants as a way of reducing global warming and protecting the environment. One solution, known as carbon sequestration, involves the capture and storage of CO2. Often the CO2 is simply buried. It would be beneficial if instead of simply burying or storing the CO2, it could be converted into another product and put to a beneficial use.
Over the years, a number of electrochemical processes have been suggested for the conversion of CO2 into useful products. Some of these processes and their related catalysts are discussed in U.S. Pat. Nos. 3,959,094; 4,240,882; 4,349,464; 4,523,981; 4,545,872; 4,595,465; 4,608,132; 4,608,133; 4,609,440; 4,609,441; 4,609,451; 4,620,906; 4,668,349; 4,673,473; 4,711,708; 4,756,807; 4,818,353; 5,064,733; 5,284,563; 5,382,332; 5,457,079; 5,709,789; 5,928,806; 5,952,540; 6,024,855; 6,660,680; 6,664,207; 6,987,134; 7,157,404; 7,378,561; 7,479,570; U.S. Patent App. Pub. No. 2008/0223727; Hori, Y., “Electrochemical CO2 reduction on metal electrodes”, Modern Aspects of Electrochemistry 42 (2008), pages 89-189; Gattrell, M. et al. “A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper”, Journal of Electroanalytical Chemistry 594 (2006), pages 1-19; and DuBois, D., Encyclopedia of Electrochemistry, 7a, Springer (2006), pages 202-225.
Processes utilizing electrochemical cells for chemical conversions have been known for years. Generally an electrochemical cell contains an anode, a cathode and an electrolyte. Catalysts can be placed on the anode, the cathode, and/or in the electrolyte to promote the desired chemical reactions. During operation, reactants or a solution containing reactants are fed into the cell. Voltage is then applied between the anode and the cathode, to promote the desired electrochemical reaction.
When an electrochemical cell is used as a CO2 conversion system, a reactant comprising CO2, carbonate or bicarbonate is fed into the cell. A voltage is applied to the cell, and the CO2 reacts to form new chemical compounds.
Several different cell designs have been used for CO2 conversion. Most of the early work used liquid electrolytes between the anode and cathode while later scientific papers discussed using solid electrolytes.
U.S. Pat. Nos. 4,523,981; 4,545,872; and 4,620,906 disclose the use of a solid polymer electrolyte membrane, typically a cation exchange membrane, wherein the anode and cathode are separated by the cation exchange membrane. More recent examples of this technique include U.S. Pat. Nos. 7,704,369; 8,277,631; 8,313,634; 8,313,800; 8,357,270; 8,414,758; 8,500,987; 8,524,066; 8,562,811; 8,568,581; 8,592,633; 8,658,016; 8,663,447; 8,721,866; and 8,696,883. In these patents, a liquid electrolyte is used in contact with a cathode.
Prakash, G., et al. “Electrochemical reduction of CO2 over Sn-Nafion coated electrode for a fuel-cell-like device”, Journal of Power Sources 223 (2013), pages 68-73 (“PRAKASH”), discusses the advantages of using a liquid free cathode in a cation exchange membrane style CO2 electrolyzer although it fails to teach a liquid free cathode. Instead, a liquid solution is fed into the cathode in the experiments discussed in PRAKASH.
In a liquid free cathode electrolyzer no bulk liquids are in direct contact with the cathode during electrolysis, however there can be a thin liquid film on or in the cathode. In addition the occasional wash or rehydration of the cathode with liquids may occur. Advantages of using a liquid free cathode included better CO2 mass transfer and reduced parasitic resistance.
Dewolf, D., et al. “The electrochemical reduction of CO2 to CH4 and C2H4 at Cu/Nafion electrodes (solid polymer electrolyte structures)” Catalysis Letters 1 (1988), pages 73-80 (“DEWOLF”), discloses the use of a liquid free cathode in a cation exchange membrane electrolyzer: an electrolyzer with a cation-conducting polymer electrolyte membrane separating the anode from the cathode. DEWOLF reports an observed maximum faradaic efficiency (the fraction of the electrons applied to the cell that participate in reactions producing carbon containing products) of 19% for CO2 conversion into useful products and a small steady state current of 1 mA/cm2.
Various attempts have been made to develop a dry cell to be used in a CO2 conversion system, as indicated in Table 1 below. However, a system in which the faradaic efficiency in a constant voltage experiment is greater than 32% has not been achieved. Furthermore, the reported rates of CO2 conversion current (calculated as the product of the faradaic efficiency for CO2 conversion and the current in the cell after 30 minutes of operation) have been less than 5 mA/cm2: too small for practical uses.
There are a few reports that claim higher conversion efficiencies. In particular, Shironita, S., et al., “Feasibility investigation of methanol generation by CO2 reduction using Pt/C-based membrane electrode assembly for a reversible fuel cell”, J. Power Sources 228 (2013), pages 68-74 (“SHIRONITA I”), and Shironita, S., et al., “Methanol generation by CO2 reduction at a Pt—Ru/C electrocatalyst using a membrane electrode assembly”, J. Power Sources 240 (2013), pages 404-410 (“SHIRONITA II”), reported “coulombic efficiencies” up to 70%. However columbic efficiency is different from faradaic efficiency. A system can have a high coulombic efficiency for the production of species adsorbed on the electrocatalyst, but may only observe a small faradaic efficiency (0.03% in SHIRONITA I and SHIRONITA II) for products that leave the catalyst layer. This phenomena is adequately explained in Rosen, B. A., et al., “In Situ Spectroscopic Examination of a Low Overpotential Pathway for Carbon Dioxide Conversion to Carbon Monoxide”, J. Phys. Chem. C, 116 (2012), pages 15307-15312, which found that when CO2 is reduced to adsorbed CO during CO2 conversion by cyclic voltammetry, most of the CO does not leave the electrolyzer.
Recently, U.S. Patent Application Publication No. US2012/0171583 (the '583 publication) disclosed a cation exchange membrane design that could be run with a liquid free cathode. The application states that a “system may provide selectivity of methanol as part of the organic product mixture, with a 30% to 95% faradaic yield for carbon dioxide to methanol, with the remainder evolving hydrogen.” However, the application does not provide data demonstrating a 30% to 95% faradaic yield. Furthermore, in trying to repeat the experiment, a steady state faradaic efficiency near zero during room temperature electrolysis was observed. These results are further laid out in Comparison Example 1 below.
In conclusion, faradaic efficiencies of less than 30% are not practical. What is needed is a process that has a faradaic efficiency of at least 50%, preferably over 80%. Furthermore, a device with a low CO2 conversion current is impractical. What is needed is a device with a CO2 conversion current of at least 25 mA/cm2.